A number of new battery chemistries are entering the market to provide capabilities required in specialized applications. At one time, the lithium-ion battery market was driven by the use of such batteries in portable electronics which require high energy but only limited life and power. More recently, other industries have focused on the use of batteries. By way of example, batteries are commonly incorporated into power tools and certain types of hybrid-electric vehicles. Each new industry requires different performance characteristics. Certain applications such as automotive applications require battery stability both in terms of battery safety for large packs and long life (at least 10 to 15 years).
Batteries with LiyFePO4 cathodes and/or Li4+xTi5O12 anodes have recently captured the attention of the automotive battery community due to their inherent stability and high rate capability. These chemistries, especially when used in combination, suffer two serious drawbacks, one drawback is the low inherent energy density of the chemistries. The energy disadvantage can be offset by the fact that these cells can generally be operated over a wider SOC range without degradation of the output of the battery. Thus, the “usable” energy may be equivalent to or greater than that of alternative chemistries for batteries of comparable size.
A second drawback results from the fact that both chemistries incorporate phase-change materials that exhibit plateaus in the range of ˜0.03<y<˜0.9 or ˜0.3<x<−2.4, resulting in a very flat open-circuit potential (OCP).
Battery state of charge (SOC) is typically estimated using a combination of two techniques: coulomb counting and OCP measurement. The former involves integrating the current that is passed to or from the cell to calculate the change in the cell's capacity. Errors in current measurement render this technique inaccurate over time, while side reactions in the cell lead to further deviations between the estimated and actual SOC. By measuring or estimating the OCP, or rest potential, of the cell, one may use OCP-SOC functional relationships to extract the SOC. The coulomb-counting technique tends to be more accurate at short times or when the current is high, while the OCP technique does better when the cell is at rest or the current is low. The two techniques of SOC estimation are typically combined in a number of different ways to obtain the most accurate estimate of SOC possible at all times.
Thus, flat or shallowly sloping OCPs, while providing some advantages, make accurate SOC estimation very difficult. Accordingly, for cells with a flat (or shallowly sloping) OCP, the OCP-SOC correlation technique does not provide the desired accuracy in determination of the cell SOC. Since coulomb counting alone is inherently inaccurate, a need exists for alternative SOC estimation techniques for systems such as the Li4+xTi5O12/LiyFePO4 cell.
What is needed therefore is a battery system and method that provides the advantages of chemistries which exhibit a flat or shallowly sloping OCP while providing a more accurate SOC determination.